In a maritime environment, submerged seismic sources, such as for example compressed-air guns, are customarily used.
The seismic wave emitted by a source submerged at a depth d propagates in the water in a substantially spherical manner. The energy sent upwards from the source is reflected specularly at the surface and is superimposed on the seismic energy sent downwards from the source. The coefficient of reflection of the seismic waves at the water-air interface is almost equal to −1, and the reflection gives rise to a change of sign of the pressure wave. That component of the emitted field which is reflected at the surface is similar, apart from the sign, to what would be emitted by a ghost source situated vertically in line with the source and at a distance d above sea level.
FIG. 1 illustrates this phenomenon, with the vertical direction z denoting the depth and the horizontal direction x a spatial coordinate parallel to the surface of the sea. The seismic signal emitted from the submerged source 10 in a direction forming an angle θ with the vertical has, at the level of a wave surface Σt some distance from the source, a direct component S(t) propagated downwards from the source 10 and a ghost component −S(t−ΔT) which has undergone the reflection on the water-air surface M as if it had been emitted from the ghost source 10′. The ghost component exhibits with respect to the direct component a delay ΔT which depends on the angle θ, i.e. ΔT=2d·cos θ/V, where V is the speed of propagation of the seismic waves in the water.
Some distance from the source, the seismic amplitude which propagates along the direction θ may be then written:S1(t)=S(t)−S(t−ΔT)  (1)
The ghost's presence related to the surface reflection affects the spectrum of the propagated seismic signal S1. If we represent an ideal source emitting a Dirac pressure pulse, that is to say with a flat spectrum, the superposition of the reflected wave brings about:                zeros or notches in the spectrum at the frequencies which are a multiple of 1/ΔT;        attenuation of the low frequencies, which is considered to be prejudicial since the information extracted from the measurements at the lowest frequencies is very rich, in particular for advising regarding the speeds of propagation of the waves in the soil.        
FIG. 2 illustrates the same phenomenon as FIG. 1 in a case where θ=0, with the depth z represented by the vertical direction and the time t represented by the horizontal direction. A Dirac pulse emitted by the source 10 arrives at a depth z under the source at an instant t, while its echo due to the ghost, of opposite sign, arrives at the same depth z with the delay ΔT.
FIG. 3 shows spectra, obtained by calculation, of the signal emitted in the vertical direction (θ=0) for source depths, d, of 5 m (curve 11), of 10 m (curve 12) and of 20 m (curve 13). The shallow sources have the advantage of rejecting the notches toward the high frequencies, while, however, attenuating the low frequencies fairly strongly.
It is possible to seek to improve the behavior at low frequencies by increasing the depth of the source. However, the notches are then at lower frequencies. Furthermore, underwater seismic sources have diminished energy efficiencies and degraded frequency contents as the depth increases, because of the effect of the hydrostatic pressure.
In order to regulate the emission spectrum, it is known to activate several sources situated at different depths. For example, in the case of FIG. 3, the activation of the three sources at depths of 5, 10 and 20 m gives rise to a spectrum represented by curve 14, resulting from the sum of the spectra represented by curves 11, 12 and 13, which shows a steeper slope at the low frequencies and zeros aligned with those of the shallowest source. This is not perfect since the resulting spectrum is not flat. However, this is a sharp improvement. A judicious choice of the depths of the combined sources makes it possible to best circumvent the notches while preserving content at the very low frequencies.
A technique making it possible to put the effect of the ghost into further perspective consists in triggering each source placed at a given depth at the moment at which the signal of the source situated just above it reaches it. Thus, the primary wave field emitted downwards is put back into phase despite different source depths. Therefore, the primary wave fields of each of the sources interfere constructively whereas this is not the case for the ghosts.
In another approach, the sources of one and the same set are grouped together in clusters each positioned at a different depth, the set of these clusters being triggered in a maximum timescale of a second, thus making it possible to preserve a stationary emission.
It has been proposed to improve the emission spectrum by disposing a screen of gas bubbles between the source and the surface so as to decrease the reflection coefficient, thereby improving the behavior at low frequencies and limiting the sagging of the spectrum in the notches. FIG. 4 thus shows the effect on the spectrum of a reflection coefficient r of 0.7. Curves 21, 22, 23 and 24 of FIG. 4 have been calculated with sources disposed like those which gave rise to curves 11, 12, 13 and 14 in FIG. 3, respectively. It is seen that the attenuation of the reflection coefficient r boosts the lowest frequencies (A). However, the drawback of this technique is that it is very complex to implement, and the improvement in performance remains limited.
A major problem encountered by all the techniques proposed to date remains the limit in terms of depth imposed on the sources, thereby greatly reducing the possibility of finding sufficient low frequencies in the signal spectrum.
An object of the present invention is to reduce the incidence of this problem and more generally to improve the spectral content of the seismic signal emitted utilized in measurements performed on the basis of one or more submerged sources.